1. Field of the Invention
This invention relates to anode materials or anode active materials for use in alkaline cells, as well as zinc alloy powders to make up said anode materials.
2. Background Information
Zinc has been favored as an anode material to be used in alkaline dry cells and other types of cells on account of its high hydrogen overvoltage and low cost. However, it has been difficult to fully suppress the evolution of hydrogen gas from the alkaline dry cells using zinc as an anode material and this has caused electrolyte leakage as a secondary problem.
The conventional methods of suppressing the evolution of hydrogen gas are mostly based on the selection of an optimal composition of zinc alloys, the application of a surface treatment to zinc particles and the addition of corrosion inhibitors to the electrolyte.
To make zinc alloys, bismuth, indium, aluminum and other elements are combined with zinc but even alloy compositions based on such combinations are not optimal for providing enhanced corrosion resistance and the zinc alloy powder which is an anode material will discharge by itself to evolve hydrogen gas during storage, whereupon the pressure in the dry cell increases to cause electrolyte leakage.
With a view to inhibiting the corrosion of zinc alloys, it has been proposed that indium compounds such as indium oxide and indium hydroxide or bismuth compounds such as bismuth oxide and bismuth hydroxide be added to a gel of the zinc powder. However, in order to have these compounds exhibit the corrosion inhibiting effect to the fullest extent, either type of compounds must be added in amounts of several hundred ppm and more but this results in overdoping and the internal resistance of the zinc alloy will so much increase as to cause adverse effects on the discharging performance of the cell.
An object, therefore, of the present invention is to provide an anode active material and an anode material for use in mercury-free alkaline manganese cells which are adapted to evolve a smaller amount of hydrogen gas than in the prior art by means of improving the physical characteristics of the corrosion inhibitor they use and by specifying the composition of the zinc alloy powder they are based upon.
The present inventors conducted intensive studies with a view to attaining the stated object. As a result, they found that by means of adding specified amounts of metals and by adding specified amounts of metallic bismuth or bismuth compounds having specified physical characteristics when preparing a gel of anode, the characteristics of the anode gel were improved to achieve a substantial reduction of hydrogen gas evolution both before and after discharge, particularly after the discharge. The present invention has been accomplished on the basis of this finding.